1. Field of the Invention
The present invention relates to a method of detecting the zero point of probe pins accurately, and a prober. Preferably, the present invention relates to probe pins zero-point detecting method which can largely reduce the needle pressure between probe pins and an object to be tested during testing, and a prober which can detect the zero point.
2. Description of the Related Art
In the process of manufacturing a semiconductor device, a prober is used to test the electrical characteristics of devices formed on a wafer. As shown in, e.g., FIGS. 9A and 9B, the prober has a loader chamber 1 which transports a wafer W, and a prober chamber 2 which tests the electrical characteristics of the wafer W moved from the loader chamber 1. The loader chamber 1 has a cassette storing portion 3, a wafer transporting mechanism 4 which transports the wafer W to the loader chamber 1, and a subchuck 5 which prealigns the wafer W with reference to the orientation flat or notch of the wafer W while the wafer transporting mechanism 4 transports the wafer W.
The prober chamber 2 has a stage 6 or which to place the wafer W which is prealigned and moved by the wafer transporting mechanism 4 from the loader chamber 1, a moving mechanism 7 which moves the stage 6 in the X, Y, and Z directions, a probe card 8 arranged above the stage 6, and an alignment mechanism 9 which accurately aligns a plurality of probe pins 8A of the probe card 8 with a plurality of electrode pads of the wafer W on the stage 6. The alignment mechanism 9 has an upper camera 9B which is attached to an alignment bridge 9A and image-senses the wafer W, and a lower camera 9C which is provided to the stage 6 and image-senses the probe pins 8A. The alignment bridge 9A can move from deep in the front surface of the prober chamber 2 to a probe center at the center along a pair of guide rails 9D. This movement is controlled to align the electrode pads of the wafer W with the probe pins 8A.
As shown in FIG. 9A, a test head T is rotatably arranged above a head plate 2A of the prober chamber 2. The test head T and probe card 8 are electrically connected to each other through a performance board (not shown). A test signal from a tester Te is transmitted to the probe pins 8A through the test head T and performance board, and is applied from the probe pins 8A to the electrode pads of the wafer W. On the basis of the test signal, the tester Te tests the electrical characteristics of the plurality of devices formed on the wafer W.
The wafer W and probe pins 8A can be aligned with each other by a conventional known method. More specifically, when the X-Y table 7 moves the stage 6 in the X and Y directions, the lower camera 9C provided to the stage 6 reaches a position immediately below a predetermined probe pin 8A. The stage 6 is vertically moved, so that the lower camera 9C image-senses the needle point of the predetermined probe pin 8A. From the position of the stage 6 at this time, the X-, Y-, and Z-position coordinates of the needle point of the probe pin 8A are calculated. Subsequently, the alignment bridge 9A advances to the probe center, to make the optical axes of the upper camera 9B and lower camera 9C coincide with each other. At this position, the upper camera 9B image-senses a predetermined electrode pad on the wafer W, to calculate the X-, Y-, and Z-position coordinates of the electrode pad. In this manner, alignment of the electrode pad on the wafer W and the predetermined probe pin 8A is ended.
A process of testing the electrical characteristics of a device formed on the wafer W after the alignment is ended will be described hereinafter. The stage 6 moves upward to a preset Z-direction position (referred to as “Z-direction alignment position” hereinafter). The wafer W is overdriven to apply a predetermined needle pressure from the probe pins 8A to the electrode pad on the wafer W. The probe pins 8A and electrode pads are electrically connected to each other. In this state, the tester Te tests the electrical characteristics of the device. After the test, the stage 6 moves downward and the test of this device is ended. The electrical characteristics of the next device on the wafer W are tested by repeating the above process.
The conventional prober can perform alignment in the X and Y directions accurately, as described above. It is, however, difficult to bring the wafer W and probe pins 8A into contact with each other at high accuracy. More specifically, the lower camera 9C image-senses the needle point of the probe pin 8A from immediately below it, to detect the distance between the needle point of the probe pin 8A and the corresponding electrode pad of the wafer W. It is difficult to detect this distance accurately, and sometimes an error occurs. Therefore, it is difficult to accurately obtain a position (referred to as “zero point” hereinafter) where the probe pins 8A and wafer W come into contact with each other almost free of a needle pressure (overdrive amount=0) on the basis of this distance. For example, FIG. 5A shows a state wherein the Z-direction alignment position of the stage 6 which has moved upward is short of size δ1 with respect to the zero point. In this state, an electrode pad P of the wafer W does not come into contact with the probe pin. Conversely, FIG. 5B shows a state wherein the Z-direction alignment position of the stage 6 has excessively above the zero point by size δ2. In this state, an excessive needle pressure acts between the probe pins 8A and electrode pads P. Hence, conventionally, the operator sets the error from the Z-direction alignment position to the zero point for each prober on the basis of his or her experience and intuition.
In the prober described in Jpn. Pat. Appln. KOKAI Publication No. 4-340734 (first to sixth lines of column [0013]), decision as to whether or not a needle (probe pin) has come into contact with the electrode of a pellet (device) is performed in the following manner. More specifically, a voltage is applied between two specific probe pins. The specific probe pins are moved close to the electrode surfaces, each covered with a metal layer such as aluminum, of an object to be tested. When the two probe pins come into contact with the electrode surfaces, a current flows between the two probe pins. This current is measured to detect the position where the probe pins come into contact with the electrodes.
In this manner, in the prober described in Jpn. Pat. Appln. KOKAI Publication No. 4-340734, that the electrodes of the device and the probe pins have come into contact with each other is determined on the basis of detection of a current flowing between the two probe pins when the electrodes of the device and probe pins come into contact with each other. An oxide film (electrical insulator) such as a native oxide film is formed on each electrode surface. When the probe pins merely come into contact with the electrode surfaces, no current flows between the two probe pins. To cause a current to flow between the two probe pins, the probe pins must be strongly urged against the electrodes. Thus, the position of the wafer obtained when the current flows between the two probe pins cannot be used as the zero point of the probe pins. Particularly, as the thickness of the wiring layer or the like of the device decreases and the number of layers in the multilayered wiring layer increases these days, the needle pressure for testing may damage the electrode pad and its underlying layer.
The present invention solves at least one of the above problems. The present invention provides probe pins zero-point detecting method which can detect the zero point of probe pins with high accuracy, and preferably which can reliably prevent any damage to the device caused by the needle pressure during testing, and a prober which can detect the zero point.
In Jpn. Pat. Appln. KOKAI Publication No. 2004-156984, the present applicant has proposed a method of detecting the contact position (zero point) of probe pins by bringing the probe pins into contact with a wafer (referred to as a “gold wafer” hereinafter) having a thin gold film surface. According to a later research, the contact resistance between the probe pins and gold wafer was unexpectedly high (see FIG. 6). Consequently, to detect the zero point by bringing the probe pins and gold wafer into contact with each other, apparently, the probe pins must be brought into contact with the gold wafer with at least a needle pressure of approximately 0.5 g/pin. In the future, the film thickness of a device will decrease, the number of layers in the device increase, and the line width of the probe pins decrease, and it will be required to bring the probe pins into contact with the electrode with a much lower needle pressure (e.g., 0.1 g/pin or less). A gold wafer cannot meet such a demand for a lower needle pressure.
The present applicant has made various studies on the conductive film of a zero-point detection plate. As shown in FIG. 6, when reduced copper or equivalent conductive metal (e.g., a copper alloy or other conductive metal) is used as the material of the conductive film, a contact resistance lower than that of gold can be obtained, which will be able to meet the demand for a lower needle pressure in the future. The present invention is based on this finding.